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This patch adds a sync integration test that exercises a problematic
scenario: the deletion of a search engine via sync while the engine is
the default one locally.

This is possible because the default search engine is sync-ed via
preferences, that is, syncer::PREFERENCES, whereas the actual list of
search engines is sync-ed via another sync datatype, namely
syncer::SEARCH_ENGINES.

Ordering across different sync datatypes is not guaranteed and
therefore, if a user changes the default search engine and deletes the
old one, it is possible that a remote sync-ing device observes this
changes in the opposite order.

There is logic in TemplateURLService to special-case this scenario to
append underscores, and this patch simply documents this behavior using
a two-client sync integration test. Because reproducing the precise
problematic ordering is not feasible, the test leverages a
newly-introduced functionality in FakeServer that allows throttling
sync datatypes (resembling a very slow sync for preferences).

Change-Id: Ia0929717d0ea99a38969298b94f68a1c1ab44364
Bug: 1022775
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2471398
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Maksim Moskvitin <mmoskvitin@google.com>
Cr-Commit-Position: refs/heads/master@{#820238}